Rendering Diffuse Color pass in Cycles
To the best of my knowledge, it is not possible to directly render only a single color pass in Cycles. However, after rendering your image, you can access the different color passes by using the compositor.

Before rendering, make sure to go to the Render>Layers menu and select "Direct", "Indirect", and "Color" for the material type you want to single out. This makes sure that you have all the necessary color passes when you get to the compositor.

In the compositor, instead of using "Image" as input to the Composite node, you want to combine in some way the three color passes "Direct", "Indirect", and "Color" for your material type. In the example below, I used two Color Overlay nodes as shown in my compositor setup.

My original image:



Diffuse Color Passes only:



Glossy Color Passes only:



My Compositor Setup: